For me: if it's not entirety - it doesn't count. Lala figured it out and I had hoped Apple will follow this path after they bought them. Sadly, they didn't.
It's not a question of what Apple figured out but what the labels would allow. Frankly, the combination of Genius suggestions and 90 second previews should be enough for most.
Last time I've checked to discover music on iTunes you had to purchase it.